The gas insulated device, such as a gas circuit breaker, gas insulated switchgear and gas insulated bus, which is used in a transmission and delivery system is structured such that a high-voltage conductor is encased in a sealed metal container filled with an insulating gas and is supported by an insulating support medium. If a defect, such as a metal particle, is present in the sealed metal container, a high electric field is formed locally at the tip of the particle, causing a partial discharge to occur. If the partial discharge continues to exist, mechanical deterioration inside the device may occur, which may eventually result in a breakdown. In view of such circumstances, to conduct preventive maintenance of gas insulated devices, early detection of partial discharge which indicates a preliminary breakdown is important.
When detecting partial discharge, a specific pattern of the partial discharge signal is developed, and based on the pattern, the type of defect is identified. In this case, the pattern of the partial discharge signal depends on the phase of the rated voltage applied to a high-voltage conductor, for example, an alternating voltage (operating voltage) of 60,000 to 500,000 volts (50/60 Hz). Therefore, the operating voltage is synchronized with the partial discharge signal (see Japanese Patent Laid-Open No. Hei 09(1997)-80111).